1. Field of the Invention
The present invention relates to improvements in counterbalanced oil well pumps, and more particularly to improvements in the structure thereof.
2. Description of the Prior Art
As the readily available oil deposits are depleted the necessity for pumping from greater depths now occurs with increasing frequency. One limiting aspect of pumping at depths greater than three thousand feet is the length and weight of the rod string which, by itself, now comprises the major component of the pump. In particular, it is the fundamental resonance of the rod string, determined by the length and the string material, that is the source of most problems. Specifically, resonance is a product of the elastic modulus and mass density of the rod string structure which in materials like steel produce extremely low fundamental resonances. These low fundamental harmonics leave little room (i.e. frequency bandpass) for any control over the stroke rate since the practicalities of pumping push the rate right up to the resonance.
It should be understood that rod string resonance, like all classical resonances, entails a phase shift of 180.degree.. Pumping at resonance will thus be effective only if the magnification factor is greater than 2, in effect doubling the elastic excursion of the rod string to produce the same flow rate. Even then, a resonance magnification factor greater than 2 is only obtainable in systems having a damping coefficient of less than 0.05, a coefficient not easily obtained in view of the many points of friction contact that may occur along any rod string. Thus deep well pumping, a problem considered herein, is not well suited for pumping at resonance, and development of techniques for raising such resonance in order to increase pumping rate are indicated.
Compounding the basic stroke rate problem are the many nonlinearities that usually occur in a pumping stroke. For example, the characteristics of any motor, whether it be electrical, hydraulic, or pneumatic, are generally non-linear, and any engagement thereof to the pump drive will necessarily involve impulse characteristics which carry the frequency components to excite the rod string. While this may be alleviated, or reduced to a great extent, by shaping the power onset impulse, there still remains the problem associated with limit nonlinearities in any pump drive. Simply, all pumping systems, whether classically linear or not, will include higher frequency components when driven to a limit. This high frequency component, if within the bandpass of rod string resonance, will excite this resonance to promote fatigue and eventually failure, without any increase in the effective work. The high cost of rod string replacement makes this a problem of first order importance.
In my prior U.S. Pat. Nos. 4,179,947 and 4,197,766 I have described a counterbalanced pumping system which, because of its features, provides well defined frequency spectra in the drive stroke and which, furthermore, may be directly mounted onto a well head. In brief terms, the foregoing system takes advantage of varying moment arms developed by wrapping flexible members around cams to produce a low frequency oscillatory mechanism. The result obtained is a long stroke, low frequency, pumping mechanism having fundamentals which are far removed from the fundamental resonance of the rod string, and which can be easily varied for optimum result. The combination of the features described in these prior patents effectively solve the problem of impulsive loading referred to above. As result of the foregoing improvements the impulse shapes entailed in applying and terminating power to the pump and at the stroke limits are both geometrically and electrically controlled, which is further optimized herein through the use of fully variable motors.
While the foregoing techniques effectively combine the efficiencies of an oscillatory system into a mechanism of a limited, well defined bandwidth, additional benefits may be obtained through the use of reinforced composite rod strings. In particular, one may note that wells are typically drilled to accommodate either a five or seven inch pipe, which by itself often limits the sectional size of the rod string. This is solved by the higher tensile strength of composites, which also offer higher resonances.
With the increased separation between the string resonance and the stroke rate with the use of variable motors it thus becomes possible to provide a control system which, both accommodates phenomena like pump off, and optimizes the pumping rate.
One should note that disruptive phenomena occurring at the lower end of the rod string is exhibited as rod string excitation. Thus impulsive loadings applied at the bottom end of the string, (normally associated with pump off or with any failure of the downhole pump plunger,) are, at best, seen indirectly at the surface as manifest amplitude changes in the rod string harmonics. Similarly, pump drive impulse at the top end of the rod string may excite these harmonics if the impulse shape includes the necessary spectra. In either case a control system set to decrease the rod string resonant energy is a control system which optimizes the rate between the flow rate limit (pump off) and the structural limit (resonance). It is such a control system that is disclosed herein.
While the prior art teaches various techniques for controlling pump rate, most require instrumenting directly the downhole pump. One may note that the rugged environment within the well bore renders any passage of instrumentation leads hazardous, and even when achieved, instrumentation signals are often insufficient to fully define the problem encountered. It is particularly significant to note that production of crude oil occurs in formations characterized by sand and debris, often entrained in the well fluid, which temporarily affect the operation of the downhole pump in the course of their passage. Simply, the operation of the downhole pump is at best "noisy". Accordingly, even fully instrumented downhole pumps require decision levels at the surface which, because of the expense of pulling an extremely long rod string for each anomalous signal, are carefully entertained. Thus it has quickly become the more preferred practice to install sufficient ruggedness in the downhole pump to ensure operation through such anomalies and which does not have to be pulled on each occurrence of a grain of sand.
Continuous or repetitive anomalies, on the other hand, cyclically load the rod string and excite harmonics resulting in exacerbation of fatigue and all efforts to reduce such prolonged cyclic loading must be undertaken in order to reduce pumping costs. Accordingly, a control system which distinguishes between the above-mentioned patterns in its response is necessary in order to maintain some reasonable returns on the equipment cost. For example, reductions in oil production as result of pump-off in the formation must be much more closely monitored in a deep well as opposed to a shallow well. Simply, the energy dissipated in various modes of a pumping stroke is substantially higher for a long rod string than it is for a rod string of less than three thousand feet. For this reason systematic improvements which combine both the improvement in rod construction and in the controls are necessary in order to optimize the pump. Simply, one has to obtain more bandwidth for any control system to operate in and once such bandwidth is achieved one must conform the control mechanism to operate in this bandwidth. It is the combination of these solutions that is described herein.